When charging a secondary battery, it is important to know how full the secondary battery is charged: to what extent of its battery capacity the secondary battery is charged during the charging process.
Conventionally, however, there was no way to know this. A conventional charger for a secondary battery stops charging the secondary battery by detecting an abnormal phenomenon (such as a rise of its temperature, its minus delta V characteristic, and the like) which occurs in the secondary battery (see, for example, patent document 1).
In this charging method, however, there is a problem that the secondary battery is overcharged, so that the internal structure of the secondary battery required for repeating charge and discharge cycles is damaged, whereby effective battery cycles of the secondary battery is decreased.
Then, in consideration of this problem, the same applicant as this application invented a charger for the secondary battery that can charge the secondary battery rapidly and accurately without damaging the secondary battery, and checks whether the secondary battery reaches its fully charged condition (i.e., the condition when its charging rate is 100%) periodically (see, patent document 2).
This charging equipment for the secondary battery charges the secondary battery, repeating by turns a main charge for letting large electric current flow through the secondary battery and a check of the fully charged condition of the secondary battery.
During the main charge, the secondary battery is charged by an applied special charging voltage higher than equilibrium voltage at full charge for a predetermined time. Special charging voltage is the voltage corresponding to the peak value of electric current outside the irreversible chemical reaction region at which the increasing rate of charging electric current to increasing applied voltage becomes smaller, and then a charging electric current does not increase any more as the secondary battery whose charging rate is approximately 0% is charged with increasing applied voltage Equilibrium voltage at full charge is the electromotive force of the secondary battery in equilibrium with the voltage supplied by the variable power supply when the variable power supply is adjusted so that the electric current detected by the current detection meter may be set to ±0 mA, where the plus terminal of the secondary battery in its fully charged condition (i.e., its charging rate is 100%) is connected to the plus terminal of the variable power supply, and the minus terminal of the secondary battery is connected to the minus terminal of the variable power supply, and the current detection meter is interposed between the secondary battery and the variable power supply.
During the check of the fully charged condition of the secondary battery, electric current flowing through the secondary battery is detected during application of the equilibrium voltage at full charge to the secondary battery for a short time, and then the secondary battery is checked whether it reaches the fully charged condition by comparing the detected electric current with the standard electric current for finishing charging.
When the detected electric current is larger than the standard electric current for finishing charging, the main charge is repeated again. Otherwise, when the detected electric current is not larger than the standard electric current for finishing charging, it is judged that the secondary battery reaches the fully charged condition and charging of the secondary battery is stopped. [Patent Document 1] The Japanese Patent Laid Open Gazette Hei. 8-9563 [Patent Document 2] The Japanese Patent Gazette No. 3430439